Problem: The scale on a map is 9cm : 8km. If two cities are 72cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 9cm on the map corresponds to an actual distance of 8km. A distance of 72cm on the map is the same as 8 $\cdot$ 9cm. The actual distance between the two cities is 8 $\cdot$ 8km, or 64km.